Printed circuit boards with electronic components mounted on the boards are ubiquitous in modern electronic apparatus. Many of the functional components of electronic circuits are now packaged in certain standard configurations. An exemplary package contains a logic or memory chip of semiconductor material encapsulated to protect it from the environment.
Such electronic circuit components very often have electrical leads protruding from the sides. These leads protrude laterally a short distance and then turn downwardly toward the face of the component to be mounted adjacent to the printed circuit board or the like. In some circuit components the leads extend straight beyond the bottom face of the component and are inserted through holes in a printed circuit board. The leads are then soldered in place in the holes.
Other components referred to as surface mounted devices have the electrical leads soldered to a conductive lines printed on the printed circuit board. Some of these leads, referred to as J leads, curl underneath the component. Other surface mounted components have leads that extend straight down to a butt joint against a line on the board. Still others have leads, referred to as gull wing, which extend beyond the lower surface of the component and then laterally to lie atop lines on the board. Some devices, known as leadless chip carriers, have grooves in the sides of the package with conductive metal filling the grooves. The metal extends far enough beneath the package to be solderable on a printed circuit board.
Although many different packages have been adopted by the electronics industry there are a lesser number of standard sizes and shapes which have been adopted by common consent for uniformity in manufacturing operations. Some of these electronic components have a row of electrical leads on each of two opposite faces of the rectangular package. This is satisfactory where about 16 to 20 electrical leads are sufficient to convey signals between the printed circuit board and the chips in the package. However, as the complexity of chips has increased, larger numbers of electrical leads have been required. It is now common to find rows of leads on all four sides of a rectangular electronic component package. For example, one such rectangular package has a total of 68 electrical leads in four rows of 17 along each of its four sides. Another has 40 leads along each side of a square package. Packages with as many as 172 leads in four rows of 43 are also in use.
Manufacturing or assembly techniques have been developed for soldering such electronic components onto printed circuit boards, whether they have through-hole connections or are surface mounted devices. It sometimes happens, however, that electronic components fail, or it is desired to salvage components from an obsolete printed circuit board. It may be desirable to remove components from a printed circuit board in order to upgrade the circuitry. A number of circumstances may make it desirable to "desolder" circuit components from a printed circuit board or the like. Quite often it is just one of a number of components that is to be removed.
Desoldering of components with electrical leads that extend through holes in the printed circuit board can often be done with little difficulty since heating from the reverse of the board can be used to melt the solder so that component can be lifted from the board. Appreciably more difficulty may be encountered when trying to desolder surface mounted components. Heat must ordinarily be applied by way of the electrical leads rather than by way of the printed circuit board.
To remove the component the separate solder joints for all of the electrical leads must be heated above the melting point of the solder so that the component can be lifted from the board without damage. If the joint for just one lead is left unmelted when the device is lifted from the board, there may be damage. It is commonly important that the "footprint" of the component on the board be left clean and reusable with minimal restoration for reuse. Heating must ordinarily be rapid to avoid overheating of the electronic component or adjacent circuit components remaining on the printed circuit board.
Further, it is often difficult to remove a single component from an array of components since modern packaging and high speed operation require that the circuit components be placed close together. This means that very little space is available for any of the instruments used for desoldering. Whatever is used for desoldering must ordinarily fit between adjacent components.
Special plier-like tools have been developed for bringing heated platens against the leads at the sides of circuit components. Such tools are held in place for a short time to permit heating and melting of the solder joints and then the component can be lifted with the special tool. These tools are somewhat costly and are therefore unsuited for operations where desoldering is only rarely required. Further, these devices are not well suited for larger sizes of components. The heating rates available are not adequate. Desoldering using commonly available soldering irons is nearly impossible since some joints freeze before others can be melted.
It is therefore, desirable to provide a technique for desoldering electronic components from printed circuit boards or the like using ordinary soldering irons, solder fountains or similar heat sources. The technique should be inexpensive for use when desoldering is only occasionally required, yet simple and rapid so that it can be adapted to repair stations where large numbers of components need to be removed. It is desirable that it be usable with infrared or hot air heating since these lend themselves to automating the desoldering operation and reducing operator error. It is particularly important that damage to printed circuit boards be avoided. It may also be desirable to reuse the component, and techniques should be used for desoldering that do not inhibit such reuse.